It is known that microsystems are often used in conditions in which the energy they need in order to operate is not available in a large quantities. This is the case notably for systems supplied by a cell or a battery. Further, for various reasons, such as ecological or economic reasons, one seeks to limit the use of such sources of energy as much as possible, which means that one seeks to extend the life span of the source of energy as much as possible, and thus to use such energy sources in conditions of optimum consumption.
Conventionally, microsystems may use a stand-by circuit which is capable of waking the system and causing it to pass from a low power mode in which only the components which must necessarily receive energy (the memories for example) have voltage applied to them, to a full power mode which enables the system in its entirety to be operated. The passage from stand-by mode to the normal operating mode is thus triggered as soon as an action is carried out on the system. A typical example of this type of operation is the personal micro-computer whose screen and other functions can be put on stand-by or be awoken, this latter operation intervening as soon as the operator touches a key of the computer keyboard.
Another more general example is that of microprocessors in all types of microsystems in which the program sequence is only triggered by an external action which causes it to pass from the execution of a stand-by program loop to the execution of the actual program itself. This example is often designated by the term "watchdog".
However, in certain cases, and notably in measuring systems, conventional stand-by/waking circuits are not appropriate. Further, certain phenomena to be measured, for example, have completely random recurrences with equally unpredictable periods of absence and presence, even though the system has to begin functioning as soon as the phenomenon reappears.
Decreasing the consumption of certain measuring systems by deliberately reducing their sensitivity vis-a-vis the phenomenon to be measured is also known. However, in this case too, the solution may not be satisfactory, for in certain cases, the sensitivity must be increased in the absence of the phenomenon, or at least maintained at its nominal value, precisely in order not to miss the moment when it reappears.